A laser guide includes a laser guide optical fiber covered with a buffer layer and a covering layer, optical connectors attached to the opposite ends of the laser guide optical fiber subjected to cutting and end surface polishing, a protection pipe covering the laser guide optical fiber, and other accessories. The laser guide optical fibers include SI type optical fibers including cores made of pure quartz glass and GI type optical fibers including cores made of quartz glass doped with germanium (Ge).
Patent Document 1 discloses a laser guide optical fiber including a core at its central portion and a clad at the outer periphery of the core, where an end surface of the optical fiber is joined to one end surface of a coreless fiber made of substantially the same material as the material of the core and having a uniform refractive index. In the optical fiber, the length of the optical path of the coreless fiber is set to be less than 1 mm so that the diameter of the beam when light transmitted through the core of the optical fiber spreads in the coreless fiber and is emitted outside from the other end surface of the coreless fiber is equal to or smaller than the outer diameter of the coreless fiber. The other end surface of the coreless fiber has a concave shape having a center of curvature on the optical axis of the optical fiber. Further, the outer diameter of the optical fiber is different from the outer diameter of the coreless fiber.
Patent Document 2 discloses that a joint end portion of a PC fiber is melted by heat treatment to form a sealing portion that seals pores of the clad, and the length L of the sealing portion is set according to calculation from the incident angle θ of signal light to the PC fiber, the outer diameter D of the fiber, the core diameter a of the fiber, the refractive index n of the sealing portion, and the like.
Patent Document 3 discloses a configuration of an optical fiber in which a coreless fiber is melted and joined to the light emitting surface of the optical fiber, and a covering material having a refractive index higher than that of the coreless fiber is provided around the coreless fiber.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-302292
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-325948
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-303166